Control system for automatic transmission

ABSTRACT

A control system for an automatic transmission, which is capable of controlling the hydraulic pressures for frictional engagement units when a predetermined shift is performed such that a first frictional engagement unit is engaged and a second frictional engagement unit is released, being connected to a power source, the revolving speed of which is temporarily raised when a predetermined shift down is performed, the control unit for the automatic transmission for a vehicle being structured such that shift which is performed by engaging and releasing the two frictional engagement units is judged, whether the judged shift is shift down in which the revolving speed of the power source is temporarily raised or shift down in which the revolving speed of the power source is not raised is judged, and the contents of control of the hydraulic pressures for the frictional engagement units are changed between shift down in which the revolving speed of the power source is temporarily raised and shift down in which the revolving speed of the power source is not raised.

This application is a Divisional of Ser. No. 08/945,271 filed Oct. 27,1997, which is a 371 of PCT/JP97/00567 filed Feb. 27, 1997.

TECHNICAL FIELD

The present invention relates to a control system for an automatictransmission, and more particularly to a system for controllinghydraulic pressure for use in so-called clutch-to-clutch shift or directpressure control in accordance with control of a power source, such asan engine or a motor.

BACKGROUND ART

Since shift of the gear stage in an automatic transmission involveschange in the rotations of a plurality of rotative elements, the inertiaforces of the rotative elements must be absorbed to make the torqueshift to be performed smoothly in order to prevent shift shock. Controlfor preventing the shift shock has been performed by controllingengaging pressure or releasing pressure for frictional engagement units,such as clutches and brakes, for performing the shift so as to absorbthe inertia forces (energy) attributable to sliding of the frictionalengagement units.

Change in the rotations of the engine which occurs when shift isperformed becomes different between a power-on state in which theaccelerator pedal has been depressed and a power-off state contrary tothe power-on state. Therefore, when clutch-to-clutch shift is performedin which the states of engagement of two frictional engagement units aresimultaneously changed, the engaging pressure for the on-comingfrictional engagement unit is made adaptable to the state of revolutionof the engine when the shift is performed.

When shift down, which is clutch-to-clutch shift, is performed, theengaging pressure for the on-coming frictional engagement unit isgradually raised (swept up) after rise in the engine revolving speed tothe synchronized revolving speed at the gear stage set by the shift downbecause the engine revolving speed is attempted to be raised in thepower-on state. In the power-off state, the engine revolving speed isundesirably lowered if the frictional engagement unit which has realizedthe gear stage is released. Therefore, the engaging pressure for theon-coming frictional engagement unit is enlarged in an early stage toraise the engine revolving speed to the synchronized revolving speed atthe gear stage set by the shift down. That is, the engaging pressure forthe frictional engagement unit is controlled to be adaptable to thetendency of the change in the revolving speed of the engine when theshift is performed.

When the clutch-to-clutch shift is performed, learning control isperformed such that the hydraulic pressure is corrected in accordancewith the state of fuel injection in the engine and the tied-up statewhen the previous shift has been performed and the shift is performedwith the corrected hydraulic pressure when the next shift is performed.

That is, the clutch-to-clutch shift is performed such that the hydraulicpressure for at least one of the frictional engagement unit of thefrictional engagement units for performing the shift is successivelychanged to correspond to the state of progress of the shift to preventshock attributable to rapid change of the output torque. In this case,change in the revolving speed (the engine revolving speed) input to theautomatic transmission is affected by the input torque, the frictioncoefficient of the frictional member or the change rate of the hydraulicpressure. Thus, there arises a possibility that fuel injection in theengine is undesirably performed excessively or a tied-up state occurs onthe contrary.

Therefore, the foregoing problems have been prevented by correcting thecontrolled value of the hydraulic pressure in accordance with thedetected state when the shift has been performed and the nextclutch-to-clutch shift is controlled in accordance with the correctedcontrolled value. Since the foregoing control is able to use theindividual difference in the automatic transmission and the factor suchas the change of the frictional engagement unit as the time lapses inthe control of the shift, control of the shift suitable for each casecan be performed. Therefore, shift shock occurring when theclutch-to-clutch shift is performed can be prevented moresatisfactorily.

Since it is preferable that rapid change in the rotations is preventedin order to prevent shift shock, a throttle valve of the engine has beenelectronically controlled to also control the engine revolving speed aswell as the control the hydraulic pressure for the automatictransmission, in recent years. An example of the foregoing structure hasbeen disclosed in Japanese Patent Laid-Open No. 5-231525 (JPA-5-231525).

The invention disclosed as described above relates to a hydraulicpressure control when so-called synchronizing shift is performed inwhich the opening of the throttle is enlarged by detecting the shiftdown when the shift down has been performed in a state where thethrottle valve is closed. Thus, the engine revolving speed issynchronized with the revolving speed at the gear stage after the shiftand shift down is performed in the foregoing state. Moreover, hydraulicpressure control means for preventing or restraining rise in the linepressure occurring attributable to the temporary enlargement of theopening of the throttle when the synchronizing shift is performed isprovided. Thus, shock occurring because of the rapid torque capacity ofthe frictional engagement unit when the shift down is performed isprevented.

When the above-mentioned clutch-to-clutch shift or direct control of thepressure is performed, initial hydraulic pressure control has beenperformed in which the hydraulic pressure which is applied to theon-coming frictional engagement unit is temporarily raisedsimultaneously or immediately after the shifted output has beenperformed to reduce a so-called pack clearance so as to cause thefrictional engagement unit to immediately be provided with a torquecapacity when higher hydraulic pressure is applied.

The initial hydraulic pressure control is a control which is capable ofbringing the frictional engagement unit into a standby state in whichthe frictional engagement unit can immediately and substantially beengaged. That is, if insufficient control is performed such that theinitial hydraulic pressure is too low, timing for the frictionalengagement unit to substantially be engaged is delayed and thus theshift response deteriorates. If the initial hydraulic pressure is toohigh, the frictional engagement unit is undesirably provided with anexcessively large torque capacity. As a result, there arises a risk thata next control of low-pressure standby cannot satisfactorily beperformed.

The above-mentioned synchronizing shift is performed at a down shift ina substantial power-off state when a manual selection by a driver iscarried out, for example. If the shift down is performed by so-calledclutch-to-clutch shift in which engagement/release states of twofrictional engagement units are simultaneously changed, the engagingpressure for the off-going frictional engagement unit is relativelyearly swept up to raise the engine revolving speed to the synchronizedrevolving speed at the gear stage after the shift. That is, inaccordance with the state of the revolution of the engine when the shiftdown has been judged, the control of the hydraulic pressure for thefrictional engagement unit is judged and performed.

On the other hand, the engine is controlled such that the revolvingspeed is raised in accordance with a fact that the shift down is thesynchronizing shift. Since the control to raise the revolving speed isperformed by temporarily opening the throttle valve, also the enginetorque is simultaneously enlarged.

In this case, the automatic transmission is controlled in accordancewith the contents of control in the power-off state and the engagingpressure for the on-coming frictional engagement unit is raised when theshift is completed. However, since the throttle opening is enlargedafter the shift has been started, a power-on state is undesirablyrealized. As a result, the control of the hydraulic pressure for theautomatic transmission and the state of the operation of the engine donot coincide with each other. Thus, the revolving speed of the engine isundesirably raised when the shift is completed, thus raising apossibility that the shift shock takes place.

The above-mentioned problem also arises when the foregoing learningcontrol of the hydraulic pressure is performed. That is, the learnedvalue of the hydraulic pressure includes the torque applied to theautomatic transmission when the previous shift has been performed.Therefore, if the hydraulic pressure for the synchronizing shift iscontrolled in accordance with the learned value of the hydraulicpressure when an ordinary shift has been performed, there arises apossibility that the shift shock takes place because the states of theinput torque are considerably different from each other.

The foregoing problems also arise when ordinary clutch-to-clutch shiftexcept for the synchronizing shift is performed. If the learned valueincludes data obtained when the synchronizing shift has been performed,the input torque at a gear shift in which the learned value is obtaineddiffers from the in-put torque at the gear shift which must becontrolled, and the learned value to be used in the gear shift isinadequate. Thus, there arises a possibility that excessive shift shocktakes place.

When the foregoing control of the initial hydraulic pressure isperformed such that the ordinary shift and the synchronizing shift arecontrolled in the same manner, the difference between the operationstate of the engine when the shift is performed and the input to theautomatic transmission may cause an inadequate initial hydraulicpressure at the gear shift. Moreover, there arises a possibility thatthe following control of the hydraulic pressure during the shift isdelayed.

An object of the present invention is to prevent shift shock inso-called synchronizing.

Another object of the present invention is to provide a control unitwhich is capable of properly controlling learning of the hydraulicpressure in a clutch-to-clutch shift.

Another object of the present invention is to provide a control unitwhich is capable of adequately controlling the initial hydraulicpressure at a clutch-to-clutch Shift or at a shift carried out bydirectly controlling the hydraulic pressure.

DISCLOSURE OF THE INVENTION

A control unit according to the present invention is arranged such thatthe contents of control of the hydraulic pressures of frictionalengagement units are made to be different from those employed when anordinary shift down operation is performed in a case where the shiftdown, in which a first frictional engagement unit is engaged and asecond means is released, is shift of a type in which the revolvingspeed of a power source is temporarily raised to about the synchronizedrevolving speed after the shift has been performed, that is, in a casewhere synchronizing shift is performed. In case that the hydraulicpressure for the first frictional engagement unit is temporarily raisedimmediately after the shifted output has been performed, the hydraulicpressure is made to be higher than that for the ordinary shift or thetime for which the raised pressure is maintained is elongated.Therefore, delay of the shift and deterioration in the durabilityoccurring attributable to sliding of the frictional engagement unit canbe prevented.

When the engaging pressure for the first frictional engagement unit isgradually raised to perform the shift in case that shift down isperformed such that the revolving speed of the power source istemporarily raised, the engaging pressure is early raised as comparedwith another shift down operation or the raising ratio is raised.Therefore, even if so-called synchronizing shift has been performed andthus the revolving speed of the power source has been raised, thehydraulic pressures for the frictional engagement units are made to besuitable to the input torque. As a result, shift shock and deteriorationin the durability of the frictional engagement units can be prevented.

When the shift down is performed by the clutch-to-clutch shift method inwhich the revolving speed of the power source is temporarily raised, thepresent invention is arranged such that the learning control of thehydraulic pressures for the frictional engagement units for performingthe shift is performed in a manner different from that for another shiftdown operation. Therefore, the controlled value obtained from thelearning control can be made to be adaptable to the input torque. As aresult, shift shock and deterioration in the durability of thefrictional engagement units can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart for explaining the contents of a control which isperformed by a control unit according to the present invention;

FIG. 2 is a time chart showing control pattern I for a third brakepressure when the gear stage is shifted down from a third speed to asecond speed in a power-on state;

FIG. 3 is a time chart showing control pattern II for the third brakepressure when the gear stage is shifted down from the third speed to thesecond speed in a power-off state;

FIG. 4 is a time chart showing control pattern III for the third brakepressure when the gear stage is shifted down from the third speed to thesecond speed in an synchronizing shift;

FIG. 5 is a graph showing an example of coordinated control of thethrottle opening and the third brake pressure when the synchronizingshift is performed;

FIG. 6 is a graph showing the relationship between the duty ratio fordetermining the initial hydraulic pressure control means and the changeratio of the engine revolving speed;

FIG. 7 is a graph showing the relationship between initial hydraulicpressure control time and the change ratio of the engine revolvingspeed;

FIG. 8 is a graph showing tendency of change of the duty ratio orconstants for use to correct the initial hydraulic pressure control timewith respect to low-pressure standby learned value;

FIG. 9 is a time chart schematically showing changes in the enginerevolving speed, the hydraulic pressure and the output torque realizedwhen clutch-to-clutch shift is performed from the third speed to thesecond speed;

FIG. 10 is a flow chart for explaining the contents of control which isperformed by the control unit according to the present invention;

FIG. 11 is a diagram showing the overall control system according to thepresent invention;

FIG. 12 is a skeleton diagram showing an example of a gear train of anautomatic transmission according to the present invention;

FIG. 13 is a table showing engagements of frictional engagement unitsfor setting gear stages in the automatic transmission;

FIG. 14 a diagram showing configuration of range positions in the shiftapparatus; and

FIG. 15 is a diagram showing a portion of a hydraulic circuit forcontrolling the engaging pressure employed when shift between the secondspeed and the third speed is performed.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described further specifically withreference to the drawings. Initially, the overall control system willnow be described. FIG. 11 shows the control system showing an engine 1and an automatic transmission 3. A signal corresponding to thedepression of an acceleration pedal 20 is supplied to an engineelectronic control unit 21. A suction duct of the engine 1 is providedwith an electronic throttle valve 23 which is operated by a throttleactuator 22. In accordance with the depression of the acceleration pedal20, a control signal is output from the engine electronic control unit21 to the throttle actuator 22 so that the degree of opening iscontrolled in accordance with the controlled variable.

There are disposed an engine revolving speed sensor 24 for detecting therevolving speed of the engine 1, an air flow meter 25 for detecting thequantity of inlet air, an inlet-air temperature sensor 26 for detectingthe temperature of the inlet air, a throttle sensor 27 for detectingopening degree θ of the electronic throttle valve 23, a vehicle speedsensor 28 for detecting vehicle velocity V in accordance with therevolving speed of an output shaft 17 or the like, a cooling-watertemperature sensor 29 for detecting the temperature of cooling water forthe engine 1, a brake switch 30 for detecting the operation of a brakeand an operation position sensor 32 for detecting the operated positionof a shift lever 31. Signals representing engine revolving speed Ne,inlet-air temperature Tha, opening θ of the electronic throttle valve23, vehicle velocity V, engine cooling water temperature THw, operationstate BK of the brake and operated position Psh of the shift lever 31are supplied from the foregoing sensors to the engine electronic controlunit 21 or the transmission electronic control unit 33. Note that thetransmission electronic control unit 33 is supplied with signalsrepresenting the opening θ of the electronic throttle valve 23 andengine cooling water temperature THw and a signal representing theoperated position Psh of the shift lever 31.

Moreover, a signal representing turbine revolving speed N_(T) issupplied from a turbine revolving speed sensor 34 for detecting therevolving speed of a turbine runner to the transmission electroniccontrol unit 33. A signal representing a kick-down operation is suppliedfrom a kick down switch 35 for detecting the operation of theacceleration pedal 20 to the maximum operation position to thetransmission electronic control unit 33. Moreover, a sports-mode switch39 which is manually operated to output a gear shift signal and asynchronizing shift switch 40 are connected to the transmissionelectronic control unit 33. An example of an apparatus of the foregoinghas been disclosed in Japanese Patent Laid-Open No. 6-307527 andJapanese Patent Application No. 7-215892.

The sports-mode switch is a switch for selecting a mode for shifting thetransmission by a manual operation or a switch for outputting atransmission signal generated in the manual operation, the sports-modeswitch being provided for a shift apparatus or an instrument panel (notshown). Structure of the foregoing type have been disclosed in, forexample, Japanese Patent Laid-Open No. 6-307527, Japanese PatentLaid-Open No. 6-48216 and Japanese Patent Laid-Open No. 6-2761. Thesynchronizing shift switch is a switch for shifting down thetransmission by one step, synchronizing shift switch being disposed atan arbitrary position, for example, in a central portion of a steeringwheel (not shown). When the transmission is shifted down by operatingthe foregoing switches, a so-called synchronizing shift control isperformed such that the electronic throttle valve 23 is opened by adegree greater than the depression of the acceleration pedal 20 inresponse to an output signal from the engine electronic control unit 21and the engine revolving speed Ne is raised to the synchronizedrevolving speed for the shifted stage after the transmission has beenshifted down. An example of the above-mentioned control has beendisclosed in, for example, Japanese Patent Application No. 7-215892.

The engine electronic control unit 21 is a so-called microcomputerhaving a central processing unit (CPU), a storage unit (RAM and ROM) andan input/output interface. The CPU uses a supplied signal in accordancewith a program previously stored in the ROM while using a temporalstorage function of the RAM to perform various engine controloperations. For example, the CPU controls a fuel injection valve 36 forcontrolling the quantity of fuel to be injected, an igniter 37 forcontrolling the ignition timing, a bypass valve (not shown) forcontrolling the idling speed and all of throttle controls including thetraction control by causing the throttle actuator 22 to control theelectronic throttle valve 23.

Also the transmission electronic control unit 33 is a microcomputersimilarly to the foregoing engine electronic control unit 21. The CPUprocesses a supplied signal in accordance with a program previouslystored in the ROM by using the temporary storage function of the RAM.Moreover, the CPU operates solenoid valves or linear solenoid valves. ina hydraulic-pressure control circuit 38. For example, the transmissionelectronic control unit 33 controls a linear solenoid valve S_(LT) forgenerating output pressure PS_(LT) corresponding to the opening of theelectronic throttle valve 23, a linear solenoid valve S_(LN) forcontrolling the back pressure for accumulators and the quantity of slipof the lock-up clutch. Moreover, the CPU operates a linear solenoidvalve S_(LU) for controlling the engaged pressure for a predeterminedclutch or a brake during shift of the transmission in accordance withthe progress of the shift of the transmission and to correspond to anapplied torque.

The transmission electronic control unit 33 judges the gear stage of theautomatic transmission 3 and an engagement state of the lock-up clutchin accordance with the standard throttle opening θ (opening of thethrottle obtained by converting the depression of the accelerator pedalwith a predetermined non-linear characteristic), the vehicle velocity Vand a shift map using the foregoing factors as parameters. To realizethe judged gear stage and the state of engagement, the transmissionelectronic control unit 33 operates No. 1 to No. 3 solenoid valvesS_(OL1), S_(OL2) and S_(OL3) of the hydraulic-pressure control circuit38. When engine brake is generated, the transmission electronic controlunit 33 operates a No. 4 solenoid valve S_(OL4).

The automatic transmission 3 according to this embodiment is structuredto be capable of setting five forward and one reverse gear stages, asshown in a skeleton diagram shown in FIG. 12. That is, as shown in FIG.12, the automatic transmission 3 is connected to the engine 1 through atorque converter 2. The torque converter 2 has a pump impeller 5connected to a crank shaft 4, a turbine runner 7 connected to an inputshaft 6 of the automatic transmission 3, a lock-up clutch 8 forestablishing the direct connection between the pump impeller 5 and theturbine runner 7 and a stator 10, the one directional rotation of whichis inhibited by a one-way clutch 9.

The automatic transmission 3 has a sub-transmission section 11 forselecting a high gear stage or a low gear stage and a main-transmissionsection 12 which is capable of selecting gear stage from the reversegear stage and the four forward gear stages. The sub-transmissionsection 11 has; a planetary gear unit 13 which is composed of a sun gearS₀, a ring gear R₀ and a pinion P0 rotatively supported by a carrier K₀and meshed with the sun gear S₀ and the ring gear R₀; a clutch C₀ and aone-way clutch F₀ disposed between the sun gear S₀ and the carrier K₀;and a brake B₀ disposed between the sun gear S₀ and a housing 19.

The main-transmission section 12 has; a first planetary gear unit 14composed of a sun gear S₁, a ring gear R₁ and a pinion P₁ rotativelysupported by a carrier K₁ and meshed with the sun gear S₁ and the ringgear R₁; a second planetary gear unit 15 composed of a sun gear S₂, aring gear R₂ and a pinion P₂ rotatively supported by a carrier K₂ andmeshed with the sun gear S₂ and the ring gear R₂; and a third planetarygear unit 16 composed of a sun gear S₃, a ring gear R₃ and a pinion P₃rotatively supported by a carrier K₃ and meshed with the sun gear S₃ andthe ring gear R₃.

The sun gear S₁ and the sun gear S₂ are integrally connected to eachother, the ring gear R₁, the carrier K₂ and the carrier K₃ areintegrally connected to one another. The carrier K₃ is connected to theoutput shaft 17. The ring gear R₂ is integrally connected to the sungear S₃. A first clutch C₁ is disposed among the ring gear R₂, the sungear S₃ and an intermediate shaft 18. A second clutch C₂ is disposedamong the sun gear S₁, the sun gear S₂ and the intermediate shaft 18.

As brake means, a band type first brake B₁ for stopping rotations of thesun gear S₁ and the sun gear S₂ is provided for the housing 19. A firstone-way clutch F₁ and a second brake B₂ are in series disposed among thesun gear S₁, the sun gear S₂ and the housing 19. The first one-wayclutch F₁ is arranged to be engaged when the sun gear S₁ and the sungear S₂ are inversely rotated opposite to the direction of rotation ofthe input shaft 6.

A third brake B₃ is disposed between the carrier K₁ and the housing 19.A fourth brake B₄ and a second one-way clutch F₂ are in paralleldisposed between the ring gear R₃ and the housing 19. The second one-wayclutch F₂ is arranged to be engaged when the ring gear R₃ is rotatedinversely. The clutches C₀, C₁ and C₂ and the brakes B₀, B₁, B₂, B₃ andB₄ are hydraulic frictional engagement units having frictional elementswhich are engaged when hydraulic pressure is applied.

The foregoing automatic transmission is able to set any one of fiveforward and reverse gear stages. The states of engagements and releasesof each frictional engagement units for setting the gear stage are shownin an engagement operation table shown in FIG. 13. Referring to FIG. 13,mark ◯ indicates an engaged state and mark × indicates a released state.

FIG. 14 shows the operated positions of the shift lever 31. Referring toFIG. 14, the shift lever 31 is supported by a supporting unit (notshown) which is capable of shifting the shift lever 31 to eightpositions combining six positions in the longitudinal direction of thevehicle and two positions in the lateral direction of the vehicle.Letter P represents a parking range position, letter R represents areverse range position, letter N represents a neutral range position,letter D represents a drive range position, numeral “4” represents a “4”range position for setting gear stages to the fourth speed, numeral “3”represents a “3” range position for setting gear stages to the thirdspeed, numeral “2” represents a “2” range position to the second speedand letter L represents a low range position for inhibiting up-shifthigher than the first speed. Note that the sports-mode switch 39 isdisposed between the “2” range position and the low range position at aposition more rearwards of the vehicle.

As shown in FIG. 13, the above-mentioned automatic transmission 3 isarranged to perform the clutch-to-clutch shift between the second speedand the third speed so that both of the engaged states of the thirdbrake B₃ and the second brake B₂ are switched. This shift must becontrolled in such a manner that the frictional engagement unitsconcerning the shift are brought to an underlap state or an overlapstate in accordance with the power on/off state or the shift up/downstate. Specifically, the hydraulic pressure for the second brake B₂ mustbe controlled in accordance with the applied torque and the hydraulicpressure for the third brake B₃ must be controlled in accordance withthe progress of the shift. Accordingly, the hydraulic-pressure controlcircuit 38 includes a circuit shown in FIG. 15 in order to smoothly andquickly perform the shift. The structure of the circuit will be brieflydescribed.

Referring to FIG. 15, reference numeral 70 represents a 1-2 shift valve,reference numeral 71 represents a 2-3 shift valve and reference numeral72 represents a 3-4 shift valve. The states of communications of portsin each shift valves 70, 71 and 72 are as indicated below the shiftvalves 70, 71 and 72. Note that the accompanying numerals represent thegear stages. Among the ports of the 2-3 shift valve 71, the third brakeB₃ is, through an oil passage 75, connected to a brake port 74 which iscommunicated with an input port 73 at the first speed and the secondspeed. An orifice 76 is interposed in the oil passage. A damper valve 77is connected between the orifice 76 and the third brake B₃. The dampervalve 77 serves as a buffer by sucking a small quantity of oil pressurein case that a line pressure has been rapidly applied to the third brakeB₃.

Reference numeral 78 represents a B-3 control valve. The B-3 controlvalve 78 directly controls the engaging pressure for the third brake B₃.The above-mentioned hydraulic pressure control is called as a directpressure control. That is, the B-3 control valve 78 has a spool 79, aplunger 80 and a spring 81 disposed between the spool 79 and the plunger80. The oil passage 75 is connected to an input port 82 which isopened/closed by the spool 79. An output port 83 selectivelycommunicated with the input port 82 is connected to the third brake B₃.The output port 83 is connected to a feedback port 84 formed at theleading end of the spool 79. On the other hand, a port 86 among theports of the 2-3 shift valve 71 which outputs a D-range pressure whenthe gear stage is not lower than the third speed is communicated throughan oil passage 87 with a port 85 opened at a position at which thespring 81 is disposed. A linear solenoid valve SLU for the lock-upclutch is connected to a control port 88 formed in an end portion of theplunger 80.

Therefore, the pressure level which is controlled by the B-3 controlvalve 78 is set in accordance with the elastic force of the spring 81and the hydraulic pressure applied to the port 85. The elastic force bythe spring 81 is enlarged in proportion to the level of the signalpressure which is supplied to the control port 88.

Referring to FIG. 15, reference numeral 89 represents a 2-3 timingvalve. The 2-3 timing valve 89 has a spool 90 having a small land andtwo lands each having a large diameter, a first plunger 91, a spring 92disposed between the spool 90 and the first plunger 91, and a secondplunger 93 disposed opposite to the first plunger 91 in such a mannerthat the spool 90 is interposed. An oil passage 95 is connected to aport 94 formed in the intermediate portion of the 2-3 timing valve 89.The oil passage 95 is connected to a port 96 among the ports of the 2-3shift valve 71 which is communicated with the brake port 74 when thegear stage is not lower than the third speed.

The oil passage 95 is branched at an intermediate position so as to be,and connected through an orifice to the port 97 opened between thesmall-diameter land and the large-diameter land. A port 98 selectivelycommunicated with the port 94 formed at the intermediate position isconnected to a solenoid relay valve 100 through an oil passage 99. Thelinear solenoid valve SLU for the lock-up clutch is connected to a portopened at an end of the first plunger 91. The second brake B₂ isconnected through an orifice to a port opened at an end of the secondplunger 93.

The oil passage 87 supplies/discharges hydraulic pressure to and fromthe second brake B₂. A small-diameter orifice 101 and an orifice 102having check ball are interposed at an intermediate position of the oilpassage 87. A large-diameter orifice 104 having a check ball which isopened when pressure is discharged from the second brake B₂ isinterposed in an oil passage 103 branched from the oil passage 87. Theoil passage 103 is connected to an orifice control valve 105 to bedescribed below.

The orifice control valve 105 is a valve for controlling pressuredischarge rate from the second brake B₂. The second brake B₂ isconnected to a port 107 arranged to be opened/closed by a spool 106 ofthe orifice control valve 105. The oil passage 103 is connected to aport 108 formed below the port 107 when viewed in FIG. 15. A port 109formed above the port 107 to which the second brake B2 is connected whenviewed in FIG. 15 is a port selectively communicated with a drain port.A port 111 of the B-3 control valve 78 is connected through an oilpassage 110 to the port 109. Note that the port 111 is a port which isselectively communicated with the output port 83 to which the thirdbrake B₃ is connected.

A control port 112 among the ports of the orifice control valve 105which is formed at an end opposite to the spring for pressing the spool106 is connected through an oil passage 113 to a port 114 of the 3-4shift valve 72. The port 114 is a port which outputs the signal pressureof the third solenoid valve S_(OL3) when the gear stage is not higherthan the third speed and which outputs the signal pressure of the fourthsolenoid valve S_(OL4). An oil passage 115 branched from the oil passage95 is connected to the orifice control valve 105 so that the oil passage115 is selectively communicated with the drain port.

A port 116 of ports of the 2-3 shift valve 71 which output the D-rangepressure when the gear stage is not higher than the second speed isconnected through an oil passage 118 to a port 117 of the 2-3 timingvalve 89 formed at a position at which the spring 92 is disposed. A port119 of ports of the 3-4 shift valve 72 which is communicated to the oilpassage 87 when the gear stage is not higher than the third speed isconnected through an oil passage 120 to the solenoid relay valve 100.

Referring to FIG. 15, reference numeral 121 represents an accumulatorfor the second brake B₂. A back pressure chamber of the accumulator 121is supplied with an accumulator control pressure regulated in accordancewith the hydraulic pressure output from the linear solenoid valveS_(LN). Note that the accumulator control pressure is controlled inaccordance with the applied torque to be raised in inverse proportion tothe output pressure from the linear solenoid valve S_(LN). Therefore,the transitional hydraulic pressure for engaging/releasing the secondbrake B₂ is shifted at higher levels in inverse proportion to the signalpressures of the linear solenoid valve S_(LN). By temporarily loweringthe signal pressure of the linear solenoid valve S_(LN), the engagingpressure for the second brake B₂ can temporarily be raised.

Reference numeral 122 represents a C-0 exhaust valve 122, and referencenumeral 123 represents an accumulator for clutch C₀. The C-0 exhaustvalve 122 acts to engage the clutch C₀ to effect the engine braking atonly the second speed in the second speed range.

Therefore, in the above-mentioned hydraulic-pressure circuit, when theport 111 of the B-3 control valve 78 is communicated with the drain, theengaging pressure for the third brake B₃ can directly be regulated bythe B-3 control valve 78. Moreover, the regulation level can be changedby the linear solenoid valve S_(LU). If the spool 106 of the orificecontrol valve 105 is positioned in the left-hand half portion of thedrawing, the second brake B₂ is communicated with the oil passage 103through the orifice control valve 105. Therefore, the pressure can bedischarged through the large-diameter orifice 104. Thus, the drainingrate from the second brake B₂ can be controlled.

The shift between the second speed and the third speed of the automatictransmission 3 is performed by the clutch-to-clutch shift such that bothof the engaging/releasing states of the second brake B₂ and the thirdbrake B₃ are simultaneously changed. When the third speed is shifteddown to the second speed, the second brake B₂ engaged at the third speedis gradually released in accordance with the input revolving speed sothat the rotation is changed. When the input revolving speed is changedto the synchronized revolving speed of the second speed, the engagingpressure for the third brake B₃ is rapidly raised at the moment at whichthe revolving speed reaches a predetermined revolving speed so that thesecond speed is realized.

As described above, the third brake B₃ must be immediately engaged inresponse to the change in the rotation occurring attributable to theprogress of the shift. On the other hand, each of general frictionalengagement units including the third brake B₃ has slight clearancebetween the friction plates and between the friction plate and thepiston of a hydraulic servo mechanism. Therefore, any torque capacity isnot provided so far as the clearance exists. Accordingly, hydraulicpressure is rapidly supplied to the engaging side frictional engagementunits simultaneously or immediately after the shifted output of theclutch-to-clutch to realize a state immediately before the engagement inwhich the torque capacity is substantially zero. That is, initialhydraulic pressure control is performed. Since the state in which theengagement is immediately established attributable to the further risein the hydraulic pressure is different depending upon the output fromthe engine or the revolving speed applied to the automatic transmission,the above mentioned control unit must perform the following control.

The engaging pressure for each frictional engagement unit of theautomatic transmission 3 is judged by the line pressure which iscontrolled in accordance with the throttle opening θ of the engine 1.For example, engaging pressure P_(B3) for the third brake B₃ when theclutch-to-clutch is performed between the second speed and the thirdspeed is controlled in accordance with the progress of the shift. Forexample, the third speed by engaging the third brake B₃. The engagingpressure P_(B3) for the third brake B₃ is quickly raised in order toraise the engine revolving speed toward the synchronized revolving speedfor the second speed in a power-off state in which the electronicthrottle valve 23 is closed when the shift is judged. In a power-onstate in which the electronic throttle valve 23 is opened, the engagingpressure is maintained at a low pressure, and then above-mentionedcontrol, rapid change in the engine revolving speed is prevented at amoment before and after the revolving speed reaches the synchronizedrevolving speed for the second speed. Thus, shift shock is prevented.

However, when the gear stage is shifted down by manually operating thesports-mode switch 39 or the synchronizing shift switch 40, the gearstage is shifted down in a state where the engine revolving speed of theengine 1 to which the automatic transmission 3 is connected has beenraised to about the synchronized revolving speed for the gear stageafter the shift down if the power-off state has been realized in whichthe electronic throttle valve 23 is closed when the shift is judged.Since the above-mentioned state in the operation is different fromeither the power-on state or the power-off state when the gear stage isshifted down from the third speed to the second speed, the followingcontrol is performed.

FIG. 1 is a flow chart showing shift down from the third speed to thesecond speed in three cases. An input signal is processed (step 1), andthen the so-called clutch-to-clutch shift is performed such that thethird speed is shifted down to the second speed (step 2). Therefore,step 2 corresponds to the shift determining means according to thepresent invention. If a negative determination is performed in step 2,any control is not performed. The operation is returned. If anaffirmative determination is performed, whether or not the present modeis the sports mode is judged (step 3). Steps 2 and 3 correspond to theshift detecting means.

As described above, the sports mode is a shift mode in which the shiftis performed in accordance with the operation of the switch. The switchis structured in such a manner that each gear stage position is providedfor a shifting apparatus and a switch, which is switched on by a shiftlevel, is provided for each gear stage position. Another structure isformed such that a sports mode state is set and, in this state, anup-shift switch or a down-shift switch is switched on by a shift lever.Another structure is formed such that an up/down switch is provided fora steering wheel or an instrument panel. Therefore, the judgment in step3 may be performed by judging whether or not an output has been madefrom the foregoing switch.

When a negative judgment is performed in step 3 because the shift downhas taken place due to change in the running condition, whether or notthe state is the power-on state is judged (step 4). That is, whether ornot the electronic throttle valve 23 has been opened and the vehicle isbeing operated by the output from the engine 1 is judged. This judgmentcan be performed in accordance with the opening θ of the throttle.

When an affirmative judgment is performed in step 4 because of thepower-on state, shift is performed by controlling release of the secondbrake B₂ and by controlling the engagement of the third brake B₃ (step5). The foregoing shift is performed because the 2-3 shift valve 71shown in FIG. 15 is switched, the linear solenoid valve S_(LU) regulatesthe engaging pressure for the third brake B₃ and thus the pressure isdischarged from the second brake B₂ in a state where the accumulator121. Since the engine 1 is in an operation state in this case, theengine revolving speed, that is, input revolving speed N_(CO) is raisedbecause the engaging pressure for the second brake B₂ state to the thirdspeed is lowered. Therefore, the engaging pressure B_(P3) for the thirdbrake B₃ for realizing the third speed is controlled in accordance withpattern I shown in FIG. 2 (step 6).

The control pattern I will be briefly described. The duty ratio of thelinear solenoid valve S_(LU) for determining the regulation level forthe third brake pressure P_(B3) is raised at time t₂ elapsed, bypredetermined time T₁, from time t₁, at which the determination of theshift from the third speed to the second speed has been established sothat the initial hydraulic pressure control is performed to reduce thepack clearance. That is, the duty ratio is set to be D₁ and this valueis maintained for T₂ seconds. Then, the duty ratio is maintained atsmall value D₂ to time t₄ at which the input revolving speed N_(CO) israised to the revolving speed which is lower than the synchronizedrevolving speed for the second speed by predetermined revolving speedΔα. Thus, the third brake pressure P_(B3) is maintained at a lowpressure. Then, the duty ratio of the linear solenoid valve S_(LU) isgradually raised so that the third brake pressure P_(B3) is graduallyraised (swept up). At time t₅ at which the input revolving speed N_(CO)has reached the synchronized revolving speed, the third brake B₃ iscompletely engaged.

As a result of the above-mentioned control, the third brake B₃substantially starts engaging when the engine revolving speed Ne hasbeen raised to the predetermined revolving speed. Thus, rapid change inthe engine revolving speed Ne before and after the moment at which itreaches the synchronized revolving speed for the second speed after theshift down can be prevented. Thus, the shift shock can satisfactorily beprevented.

When the gear stage is shifted down in the power-on state, a control isperformed such that the engine torque is reduced by delaying theignition timing or by temporarily raising the releasing pressure for thesecond brake B₂ on the releasing side.

If a negative judgment is performed in step 4 because of the power-offstate, the gear stage is shifted down by releasing the second brake B₂and by engaging the third brake B₃ in the power-off state (step 7).Moreover, the engaging pressure for the third brake B₃ is controlled inaccordance with pattern II shown in FIG. 3 (step 8).

That is, the initial hydraulic pressure control is performed such that astate in which the duty ratio of the linear solenoid valve S_(LU) is setto D₁ is maintained for T₂ seconds. After the end time t₃, the dutyratio of the linear solenoid valve S_(LU) is gradually raised at time t₆at which the input revolving speed N_(CO) is considerably lower than thesynchronized revolving speed for the second speed so that the engagingpressure P_(B3) for the third brake B₃ is gradually raised. After timet₇ at which the third brake pressure P_(B3) has been sufficientlyraised, the input revolving speed N_(CO) reaches the synchronizedrevolving speed for the second speed. Therefore, when the gear stage isshifted down from the third speed to the second speed in the power-offstate, the engaging pressure P_(B3) for the third brake B₃ is earlyraised so that the revolving speed of the engine 1 is raised by thetorque applied from the output shaft to quickly perform the shift.Moreover, the input revolving speed N_(CO) is smoothly changed to thesynchronized revolving speed so that the shift shock is satisfactorilyprevented. Since the third brake B₃ is, in this case, maintained in thestate in which it can immediately be accordance with the input revolvingspeed, undesirable shift shock does not take place. Since the state isthe power-off state in this case, the delay control of the ignitiontiming and the control for temporarily raising the releasing pressurefor the second brake B₂ on the releasing side to reduce the outputtorque are not performed.

If an affirmative judgment is performed in step 3 because of the shiftdown in the sports mode, synchronizing shift is performed (step 9).Therefore, step 3 corresponds to the synchronizing shift determiningmeans according to the present invention. The synchronizing shift isshift control in which the engine revolving speed Ne is raised to thesynchronized revolving speed in the realized gear stage after the shiftdown and the shift down is performed in this state. Therefore, theelectronic throttle valve 23 is temporarily opened by the engineelectronic control unit 21 to correspond to the release of the secondbrake B₂ and the engagement of the third brake B₃.

Moreover, coordinated control of the opening θ of the throttle and thethird brake pressure P_(B3) is performed (step 10). Then, the thirdbrake pressure P_(B3) is controlled in accordance with pattern III shownin FIG. 4 (step 11). Note that steps 6, 8 and 11 correspond to theengagement control changing means according to the present invention.

The coordinated control in step 10 is control in which the throttleopening θ and the third brake pressure P_(B3) are relatively changed inorder to cause the engine revolving speed Ne to be changed smoothly withrespect to the synchronized revolving speed for the second speed whenthe gear stage is shifted down to the second speed in a state where theopening θ of the throttle is enlarged. The control is performed in sucha manner that either or both of the throttle opening e and the thirdbrake pressure P_(B3) are arbitrarily changed. Therefore, step 10corresponds to the coordinated control means according to the presentinvention.

The specific example of the coordinated control will be now described.FIG. 5 shows an example in which the throttle opening is changed in sucha manner that the duty ratio of the linear solenoid valve S_(LU) israised in a stepped manner so that the engaging pressure P_(B3) is sweptup. Simultaneously, the throttle opening θ is reduced so that the rateof the enlargement of the engine revolving speed Ne is gradually loweredas indicated by a continuous line shown in FIG. 5 so as to be smoothlychanged to the synchronized revolving speed for the second speed. As aresult, torsion in the power transmission system and shock attributableto the torsion can be prevented. Note that change in the throttleopening θ is not required to be always performed simultaneously withsweeping up to the third brake pressure P_(B3). The change may beperformed at arbitrary time before and after sweeping up. As describedabove, the change rate of the third brake pressure P_(B3) may be changed(lowered) by changing the step width of the duty ratio in addition tothe change in the throttle opening θ.

The pattern III will be briefly described. The duty ratio of the linearsolenoid valve S_(LU) in the initial hydraulic pressure control is setto be value D₃ judged on the basis of value D₁ in substantially anordinary state and larger than the value D₁ so that the hydraulicpressure which is supplied to the third brake B₃ is raised. When theduty ratio D₁ has been changed because of learning, also the duty ratioD₃ may synchronously be changed at the same rate. Therefore, even if theinput revolving speed is raised, the third brake B₃ can quickly be setto a state immediately before the satisfactory engagement correspondingto the input revolving speed so as to be brought to a standby state in alow pressure state. Then, the duty ratio is lowered to predeterminedvalue D₂ so that the third brake pressure P_(B3) is brought to thestandby state at a low pressure. Since the throttle opening has beenenlarged, the duty ratio is raised in a stepped manner at time t₈ atwhich the engine revolving speed Ne (the input revolving speed N_(CO))has reached revolving speed which is lower than the synchronizedrevolving speed for the second speed by predetermined revolving speedΔβ(>Δα) so that the third brake pressure P_(B3) is raised. The dutyratio is raised stepwise, the width of each of which is made to belarger than that in the power-on state. Therefore, the ratio of rise inthe third brake pressure P_(B3) is raised as compared with that in theshift down in the power-on state. That is, step 10 corresponds to theinitial hydraulic pressure control means according to the presentinvention.

Therefore, even if the synchronizing shift is performed in which theengine revolving speed is raised during shift, the initial hydraulicpressure control is completed more quickly as compared with the ordinarystate. Therefore, sweeping up of the third brake pressure P_(B3) is notdelayed. As a result, even if sweeping up is early performed or even ifa period in which standby at low pressure is not substantiallyperformed, the shift control can satisfactorily be performed.

The control which is performed in accordance with the pattern III isarranged in such a manner that the engaging pressure P_(B3) for thethird brake B₃ which is the on-coming frictional engagement unit isswept up earlier than the case of the power-on down shift in the case ofthe synchronizing shift in which the throttle opening is smaller thanthe power-on state or the sweeping up ratio is raised. Therefore, shiftcan quickly be performed and the input revolving speed N_(CO) (theengine revolving speed) can smoothly be changed to the synchronizedrevolving speed for the second speed. As a result, shift shock cansatisfactorily be prevented.

Since the initial hydraulic pressure control is control for moving thefriction plate of the frictional engagement unit to a state immediatelybefore the engagement, time for which the initial hydraulic pressure issupplied may be elongated to correspond to the state in which the inputrevolving speed has been raised. An example of this case is indicated bya broken line shown in FIG. 4 in which a predetermined duty ratio D₁ ismaintained for T₃ (>T₂) seconds. In this case, T₃ is judged on the basisof T₂. If T₂ is changed due to learning or the like, T₃ may be changedat a predetermined ratio corresponding to the change.

When the hydraulic pressure for use in the initial hydraulic pressurecontrol or the time for which the hydraulic pressure is maintained ischanged to be adaptable to the synchronizing shift, the duty ratio D₃and time T₃ may be judged as shown in FIGS. 6 to 8. That is, theforegoing values are set to be the functions of the change ratio (Nedots) of the engine revolving speed Ne and their coefficients k1 and k2and constants a and b are corrected in accordance with the valueslearned (learning is performed after the shift has been performed) inthe state of standby at low pressure. FIG. 8 shows general tendency ofthe constants a and b with respect to the learned value of the standbypressure at low pressure. That is, the initial hydraulic pressurecontrol time is elongated or the hydraulic pressure is raised inproportion to the change ratio of the engine revolving speed.

Since the correction of the initial hydraulic pressure control isperformed when the synchronizing shift is performed, judgment step (step3′) for judging whether or not the shift is the shift down performed bythe synchronizing shift switch may be substituted for step 3′ shown inFIG. 1.

When the gear stage is shifted down from the third speed to the secondspeed, the second brake pressure P_(B2) and third brake pressure P_(B3)are controlled in accordance with the learned values. That is, the thirdspeed is set such that the second brake B₂ is engaged. In response tothe shift signal, discharge of pressure is started from the second brakeB₂. The second brake pressure P_(B2) is feedback-controlled inaccordance with the engine revolving speed Ne when the back pressure forthe accumulator 121 is controlled by the linear solenoid valve S_(LN).The control is continued from time t₁ at which shifted output isperformed to time t₁₂ at which the engine revolving speed Ne is raisedto the synchronized revolving speed for the second speed. Then learningcontrol is performed in which the controlled value of the back pressurefor the accumulator 121 with respect to the input torque at the controlis judged in accordance with the variation in the feedback quantity.That is, when the gear stage is next shifted down from the third speedto the second speed with the input torque, the learned controlled valueis employed to control the back pressure for the accumulator, that is,the controlled value for the releasing pressure P_(B2) for the secondbrake B₂. Note that control of the foregoing type has been disclosed in,for example, Japanese Patent Laid-Open No. 1-150050 and Japanese PatentLaid-Open No. 63-291738.

On the other hand, initial hydraulic pressure control (quick up) of thethird brake B₃ is performed at time t₁₁ to reduce the pack clearance.Thus, the third brake B₃ is maintained at a low pressure (brought to astandby state) from time t₁₃ at which the initial hydraulic pressure isended to time t₁₄ at which the engine revolving speed Ne reaches apredetermined revolving speed. Then, the duty ratio of the linearsolenoid valve S_(LU) is raised stepwise so that the engaging pressureP_(B3) for the third brake B₃ is swept up and first applied to time t₁₂at which the engine revolving speed Ne has reached the synchronizedrevolving speed. Thus, the engaging pressure is rapidly raised. The lowstandby pressure during the foregoing control is learning-controlled.Control of the foregoing type has been disclosed in, for example,Japanese Patent Laid-Open No. 6-331016.

The engine revolving speed Ne is, when the gear stage is shifted,changed in accordance with the engine torque and the engaging forces ofthe brakes B₂ and B₃. Therefore, the learned value is obtained for eachthrottle opening. However, if the above-mentioned synchronizing shift isperformed, the electronic throttle valve 23 is opened when the shift hasbeen started and thus the output from the engine is enlarged. Therefore,the engine torque is made to be different from that in the case of theshift down in which the control of the synchronizing shift is notperformed. Thus, the previous learned value is not suitable.Accordingly, the control unit according to the present inventionperforms the learning control as follows.

FIG. 10 is a flow chart of the shift down operation from the third speedto the second speed in which the operation is classified into threestates. After an input signal is processed (step 20), shift down fromthe third speed to the second speed, which is a so-calledclutch-to-clutch shift, is judged (step 21). If a negative judgment isperformed in step 21, any special control is not performed and theoperation is returned. If an affirmative judgment is performed, whetheror not the mode is the sports mode is judged (step 22).

If a negative judgment is performed in step 22 because of the shift downwhich is performed because the running condition has been changed,whether or not the state is the power-on state is judged (step 23). Ifan affirmative judgment is performed because of the power-on state,shift is, as described above, performed by controlling the release ofthe second brake B₂ and by controlling the engagement of the third brakeB₃ (step 24).

Foregoing steps 20 to 24 are the same as steps 1 to 5 shown in FIG. 1.

If the shift down is performed in the power-on state, control of thelinear solenoid valve S_(LU) and control of the back pressure for theaccumulator 121 by the linear solenoid valve SLN are performed inaccordance with the learned value as described above. During the shift,learning of the hydraulic pressure for each of the brakes B₂ and B₃ isperformed and the learned value is stored (step 25). In this case, thelearned values are stored in the form of controlled values or in theform of, for example, a map composed of corrected values of thecontrolled values. The learned values are stored and used as learnedvalues for the shift down in the power-on state and for the shift modewhich is not the synchronizing shift.

If a negative judgment is performed in step 23 because of the power-offstate, shift down in the power-off state is performed (step 26). Also inthis case, the second brake pressure P_(B2) is controlled byfeedback-controlling the back pressure for the accumulator 121 inaccordance with the characteristic of the accumulator 121 so as to bechanged as shown in FIG. 9. Step 26 above is the same as step 7 shown inFIG. 1.

As described above, the control of the linear solenoid valve S_(LU) andthe control of the back pressure for the accumulator 121 by the linearsolenoid valve SLN are performed in accordance with the learned values.During the shift, the hydraulic pressure for each of the brakes B₂ andB₃ is learned so as to be stored as learned value (step 27). In thiscase, the learned values are formed in the form of controlled values orin the form of a map composed of corrected values of the controlledvalues. The learned values are stored and used as learned values for theshift down in the power-off state and for a shift mode which is not thesynchronizing shift. That is, the learning control is performedindividually from that in the power-on state and the synchronizing shiftto be described later.

If an affirmative judgment is performed in step 22 because of the shiftdown in the sports mode, the synchronizing shift is performed (step 28).Control in step 28 is the same as step 9 shown in FIG. 1.

The hydraulic pressure for each of the brakes B₂ ad B₃ during thesynchronizing shift is learning-controlled (step 29). In this case, thelearned values are stored as controlled values or in the form of, forexample, a map composed of corrected values of the controlled values.Since the shift is the synchronizing shift in which the engine revolvingspeed Ne is raised to the synchronized revolving speed, the learnedvalues are stored and used individually from the shift in the power-onstate or the power-off state. The learning control is performedindividually from that in the power-on state and the power-off state.That is, step 29 corresponds to the learning control changing meansaccording to the present invention.

Since the synchronizing shift is not limited to the shift down in thesports mode and is performed by switching the synchronizing shift switchon, the judgment step (step 22′) for judging whether or not the shiftdown is the shift down in the synchronizing shift may be substituted forstep 22 shown in FIG. 10.

Although the foregoing embodiment is structured such that the learningcontrol of the hydraulic pressure when the synchronizing shift isperformed is performed individually from the learning control of thehydraulic pressure in the power-on state or the power-off state, thepresent invention may be structured such that the learning control ofthe hydraulic pressure in the synchronizing shift is further changed inaccordance with the degree of rise in the revolving speed of the powersource, for example, the engine. In this case, a structure may be formedsuch that the change rate (rise rate) is detected from a detected valueof the revolving speed of the power source after the determination ofthe synchronizing shift has been performed. Then, the learning controlof the hydraulic pressure is individually performed for each of resultsof the detection. Specifically, the structure is formed into a controlunit for an automatic transmission characterized in that the learningcontrol of the hydraulic pressure is changed to correspond to the degreeof rise in the revolving speed in a case of the shift in which therevolving speed of the power source is temporarily raised. Although thedescription of the above-mentioned embodiment has been performed aboutthe shift down from the third speed to the second speed, the presentinvention is not limited to the above-mentioned embodiment. The presentinvention may be applied to an apparatus for controlling shift down toanother gear stage or an apparatus for directly controlling thehydraulic pressure for the frictional engagement unit by a linearsolenoid valve or the like. Therefore, the frictional engagement unit,the engaging pressure of which including the initial hydraulic pressuremust be controlled, may be a frictional engagement unit except for thesecond and third brakes. The present invention is characterized bycontrolled contents peculiar to the synchronizing shift. Therefore, thecontrolled contents are not limited to the control pattern shown in FIG.4 and the contents may arbitrarily be changed. The present invention maybe embodied in an automatic transmission or its control unit having agear train and a hydraulic-pressure circuit different from those shownin FIGS. 12 and 15. Note that the power source may be another poweroutput unit, such as an electric motor, which is employed in place ofthe engine.

The advantages of the present invention will synthetically be described.According to the present invention, when the revolving speed of a powersource is temporarily raised even in a case of a shift down in apower-off state, the hydraulic pressure for the frictional engagementunit is controlled in a manner different from that in the case of theordinary power-off shift down. Therefore, the hydraulic pressure for thefrictional engagement unit is made to be adaptable to the input torque.As a result, adverse shift shock and deterioration in the durability ofthe frictional engagement unit can reliably be prevented.

Since the present invention is structured such that the initialhydraulic pressure is controlled to correspond to temporal rise in therevolving speed of the power source in a case of a so-calledclutch-to-clutch down shift, delay of the shift, undesirable shift shockand deterioration in the durability of the frictional engagement unitcan be prevented.

Moreover, the present invention, structured such that the hydraulicpressure for the frictional engagement unit at the final stage of theshifting operation is controlled to correspond to the rise in therevolving speed of the power source, is able to prevent shock occurringattributable to torsional vibrations of the power transmission system.In particular, a significant effect can be obtained in a case where therevolving speed of the power source and the hydraulic pressure for thefrictional engagement unit are controlled in a coordinated manner.

When the hydraulic pressure for the frictional engagement unit islearning-controlled, the present invention is structured in such amanner that different learning controls are performed between the casewhere the revolving speed of the power source is temporarily raised andthe case where the same is not raised even if the power-down shift isperformed. Therefore, the hydraulic pressure for the frictionalengagement unit can furthermore appropriately be controlled with respectto the input torque. As a result, shift shock can be prevented and thedurability of the frictional engagement unit can be improved.

What is claimed is:
 1. A control system for an automatic transmission,which is capable of controlling the hydraulic pressures for frictionalengagement units when a predetermined shift is performed such that afirst frictional engagement unit is engaged and a second frictionalengagement unit is released, being connected to a power source, therevolving speed of which is temporarily raised when a predeterminedshift down is performed, comprising: shift judging means for judgingshift which is performed by engaging and releasing said two frictionalengagement units; synchronizing shift judging means for judging whethershift judged by said shift judging means is shift down in which therevolving speed of said power source is temporarily raised or shift downin which the revolving speed of said power source is not raised;hydraulic-pressure control changing means for changing the contents ofcontrol of the hydraulic pressures for said frictional engagement unitswhen a shift judged by said shift judging means is performed between thecase where the shift down judged by said synchronizing shift judgingmeans is the shift down in which the revolving speed of said powersource is temporarily raised and the shift down in which the revolvingspeed of said power source is not raised; initial hydraulic-pressurecontrol means for raising engaging pressure for said first frictionalengagement unit immediately after the shifted output has been performedwhen the predetermined shift is performed and maintaining the engagingpressure at a low level, wherein said hydraulic pressure control meansincludes initial hydraulic pressure changing means for changing thecontents of control of the engaging pressure immediately after theshifted output has been performed under control of said initialhydraulic pressure control means; low-pressure standby learning meansfor learning and controlling the length of time for which the pressureis maintained at the low level; and means for changing the pressurewhich is raised immediately after the output for shift has beenperformed to correspond to change in the pressure maintained at the lowlevel in case that the pressure maintained at the low level has beenchanged by said low-pressure standby learning means.
 2. A control systemfor an automatic transmission according to claim 1, wherein said meansfor changing time includes means for elongating the length of time tocorrespond to a change rate of the revolving speed when the revolvingspeed of said power source has been temporarily raised.
 3. A controlsystem for an automatic transmission, which is capable of controllingthe hydraulic pressures for frictional engagement units when apredetermined shift is performed such that a first frictional engagementunit is engaged and a second frictional engagement unit is released,being connected to a power source, the revolving speed of which istemporarily raised when a predetermined shift down is performed,comprising: shift judging means for judging shift which is performed byengaging and releasing said two frictional engagement units;synchronizing shift judging means for judging whether shift judged bysaid shift judging means is shift down in which the revolving speed ofsaid power source is temporarily raised or shift down in which therevolving speed of said power source is not raised; hydraulic-pressurecontrol changing means for changing the contents of control of thehydraulic pressures for said frictional engagement units when a shiftjudged by said shift judging means is performed between the case wherethe shift down judged by said synchronizing shift judging means is theshift down in which the revolving speed of said power source istemporarily raised and the shift down in which the revolving speed ofsaid power source is not raised; initial hydraulic-pressure controlmeans for raising engaging pressure for said first frictional engagementunit immediately after the shifted output has been performed when thepredetermined shift is performed and maintaining the engaging pressureat a low level, wherein said hydraulic pressure control means includesinitial hydraulic pressure changing means for changing the contents ofcontrol of the engaging pressure immediately after the shifted outputhas been performed under control of said initial hydraulic pressurecontrol means; low-pressure standby learning means for learning andcontrolling the pressure which is maintained at the low level, and meansfor changing the pressure which is raised immediately after the outputfor shift has been performed to correspond to change in the pressuremaintained at the low level in case that the pressure maintained at thelow level has been changed by said low-pressure standby learning means.4. A control system for an automatic transmission according to claim 3,wherein said means for changing the pressure includes means for raisingthe pressure to correspond to the change rate of the revolving speedwhen the revolving speed of said power source is temporarily raised. 5.A control system for an automatic transmission, which is capable ofcontrolling the hydraulic pressures for frictional engagement units whena predetermined shift is performed such that a first frictionalengagement unit is engaged and a second frictional engagement unit isreleased, being connected to a power source, the revolving speed ofwhich is temporarily raised when a predetermined shift down isperformed, comprising: shift judging means for judging shift which isperformed by engaging and releasing said two frictional engagementunits; synchronizing shift judging means for judging whether shiftjudged by said shift judging means is shift down in which the revolvingspeed of said power source is temporarily raised or shift down in whichthe revolving speed of said power source is not raised;hydraulic-pressure control changing means for changing the contents ofcontrol of the hydraulic pressures for said frictional engagement unitswhen a shift judged by said shift judging means is performed between thecase where the shift down judged by said synchronizing shift judgingmeans is the shift down in which the revolving speed of said powersource is temporarily raised and the shift down in which the revolvingspeed of said power source is not raised; initial hydraulic-pressurecontrol means for raising engaging pressure for said first frictionalengagement unit immediately after the shifted output has been performedwhen the predetermined shift is performed and maintaining the engagingpressure at a low level, wherein said hydraulic pressure control meansincludes initial hydraulic pressure changing means for changing thecontents of control of the engaging pressure immediately after theshifted output bas been performed under control of said initialhydraulic pressure control means; wherein said hydraulic pressurecontrol means includes learning-control changing means for changing thecontents of the learning control of the hydraulic pressure for saidfrictional engagement unit between the case of the shift down in whichthe revolving speed of said power source is temporarily raised and thecase of the other shift down.
 6. A control system for an automatictransmission according to claim 5, wherein said learning-controlchanging means includes means for learning and storing the hydraulicpressure for said frictional engagement unit when the shift down inwhich the revolving speed of said power source is temporarily raised isperformed and means for learning and storing the hydraulic pressure forsaid frictional engagement unit when the downshift in which therevolving speed of said power source is not raised is performed.